Wireless communication is well known in the art. Heretofore, one type of wireless communication is known as a "cellular" communication wherein each stationary unit receives and transmits signals to mobile units within its allocated geographical region, called a cell. As mobile units move from one cell to another, communication is transferred from one stationary unit in one cell to another stationary unit in another cell.
Heretofore, cellular communication is analog based and has risen in popularity. However, as a result, the airways have become increasingly crowded and the capacity of the communication system to take on new subscribers is becoming increasingly of a problem. Digital cellular communication offers an opportunity to increase the number of subscribers to operate within the cellular system.
One of the problems of a digital wireless communication system is the non-linearity of the channel. Another problem is the equalizing of the digitally encoded signals. As the digitally encoded signal is transmitted from one unit to another, through a multiplicity of data paths, the various signals arriving at the other unit can cause delay spread between the digitally encoded signals. This produces inter-symbol interference. An equalizer is a digital hardware/software apparatus which corrects inter-symbol interference between the digitally encoded signals arriving from a plurality of signal paths.
In the prior art, a number of equalization strategies is disclosed. See, for example, "BER Performances Of Mobile Radio Equalizer Using RLS Algorithm In Selective Fading Environment" by Akihiro Higashi, Hiroshi Suzuki; "Bi-Directional Equalization Technique For TDMA Communication Systems Over Land Mobile Radio Channels" by Yow-Jong, Liu, page 1458-1462, Globecom '91; and "Development Of Japanese Adaptive Equalization Technology Toward High Bit Rate Data Transmission In Land Mobile Communications" by Seiichi Sampei, page 1512-1521 IEICE Transactions, Volume E, 74, No. 6, June, 1991.
In a multi-path fading environment the signal arriving at a receiver is composed of several signals, each of which corresponds to the transmitted signal following a distinct path from the transmitter to the receiver. In a time varying channel, the multi-path combinations of the transmitted signal at the receiver produce a signal whose amplitude is time varying and experiences fading due to destructive combining of the received signals. For digital transmission, the multi-path fading of the channel results in substantially larger average values of the Bit Error Rate (BER) when compared to a non-faded channel operating at the same average Signal to Noise Ratio (SNR). Therefore, to achieve a given average BER value in a fading channel the required SNR value is substantially higher than that required in a non-faded channel.
In environments where the modulated carrier is subject to fast multi-path fading, conventional continuous time synchronization techniques cannot be performed satisfactorily. In time domain multiple access systems where each user is assigned one time slot the receiver has to perform its timing and frequency synchronization in a slot by slot basis. Frequency synchronization is usually achieved by automatic frequency control circuits which have some inherent frequency error. Therefore, the timing recovery circuit or algorithm has to accommodate some limited amount of frequency error without significant degradation in performance.
Heretofore, to reduce the effects of fading, i.e. to reduce the required SNR values for given BER targets, signal diversity at the receiver has been considered. A receiver is provided with two or more independently faded versions of the same transmitted signal. By so doing, the probability that all the faded signals suffer large attenuations simultaneously is reduced resulting in lower detection error probability.
Several timing recovery techniques have been disclosed in the prior art. They can be classified into four categories. In the first category the threshold crossings of the received baseband data signal are compared with the sampling phase. A correction of the sampling phase is initiated as a result of this comparison. The main location of the crossings is estimated and the optimum sampling instant is assumed to be halfway between these crossings. The second technique uses the spectral line at the clock frequency or multiple of this frequency. This frequency is filtered out with a narrow band filter. The third technique is the sample-derivative system. In this technique a sampled-derivation phase detector which generates an error signal during each symbol interval proportional to the time derivative of the signal at the sampling time multiplied by the signal polarity at that time is used. The sampling derivative timing recovery system attempts to set the sampling time to coincide with the peak of the signal. Finally, in the fourth technique, a bank of all pass filters is used as a timing phase detector. This technique is suitable for the signals whose frame structure contains a synchronization field.
Fast multipath fading severely degrades the average BER performance of digital land mobile radio transmission systems. In order to achieve highly reliable digital data transmission without excessively increasing both transmitter power and co-channel reuse distance, it is well known to use diversity reception.
A diversity technique requires a number of signal transmission paths, named diversity branches, all of which carry the same information but have uncorrelated multipath fadings, and a circuit to combine the received signals into one which can be decoded reliably. Depending upon the land mobile radio propagation characteristics, there are a number of methods to construct diversity branches. Generally, they are classified into the following five categories: (1) space, (2) angle, (3) polarization, (4) frequency, and (5) time diversity.
Space diversity, which has been the most widely used because it can be implemented simply and economically, comprises a single transmitting antenna and a number of receiving antennas. Space between adjacent receiving antennas is chosen so that multipath fading appearing in each diversity branch becomes uncorrelated with that of the other branch.